The present invention generally relates to processing of material in a manufacturing plant and, more particularly, to methods and systems for tracking the movement of wafers in a semiconductor processing plant.
Conventional manufacturing plants move material to be processed through a manufacturing process having several processing areas. Currently these material lots are tracked in larger quantities that may be disposed in a carrier for ease of movement throughout the facility.
Some manufacturing processes require that the item being processed be rotated regularly in order to ensure that the item is properly processed, such as when painting an object or when applying a coating to a substrate. In the case of a mechanical process, the object is rotated to ensure that the tooling is being worn evenly or that the tooling is mechanically treating the object evenly. Even though some of these items may be individually processed, or processed in small lots, the items may form part of a larger lot being manufactured and it is difficult to distinguish the progress of the individual item as it moves through the processing line. As the number of processing steps increase tracking becomes even more difficult. This is particularly a problem in the processing of wafers in a semiconductor processing plant.
A conventional semiconductor fabrication plant typically includes multiple fabrication areas or bays interconnected by a path, such as a conveyor belt. Each bay generally includes the requisite fabrication tools (interconnected by a subpath) to process semiconductor wafers for a particular purpose, such as photolithography, chemical-mechanical polishing or chemical vapor deposition, for example. Material stockers or stocking tools generally lie about the plant and store semiconductor wafers waiting to be processed. Each material stocker typically services two or more bays and can hold hundreds of cassettes. The wafers are usually stored in cassettes in-groups of about 25 wafers. The wafers are then disposed within a carrier and move from one process step to another in the carrier. The carriers are usually tracked by their carrier code by a computer system as they move through the plant.
Once a lot has been retrieved, and the equipment has been set up, the operation on the wafers by a particular piece of equipment, or xe2x80x9ctool,xe2x80x9d can begin. At this point, the lot is xe2x80x9cmoved-inxe2x80x9d to the operation. An operator on the line then communicates this information to the host computer. The lot remains in this state until the operation is completed. Once the operation is completed, the operator must perform tests and verifications on the wafers. When all tests and verifications have been performed, the host computer application program must be notified. Wafers may have moved from one cassette to another as a result of the operation; therefore the host application and computer have to be notified of these moves. The operator then places the cassette of xe2x80x9cmoved-outxe2x80x9d wafers in the material stocker to await orders as to the location of the next piece of equipment that will perform operations on the wafers.
The semiconductor fabrication plant, including the bays, material stockers and the interconnecting path, typically operates under control of a distributed computer system running a factory management program. In this environment, the automated material handling system (AMHS) may conceptually include the cassettes, the transportation system (e.g., paths) and control system (e.g., the distributed computer system). An empty carriers management system as well as a separate test wafer management system may also form part of the AMHS.
Data gathered during the course of wafer processing is used to diagnose yield problems and forms the basis of yield improvement efforts. Such data includes parametric electrical test data gathered on individual circuits and test structures fabricated on the wafers, as well as wafer sort data which tests the suitability for use of the wafers once wafer processing is completed. One of the possible sources of yield variation is the order in which wafers in a lot are processed at a given processing step. When the processing is done one wafer at a time per step, yield variation may occur due to a build up of contaminants, uneven heating of a processing chamber or another physical aspect that changes during the processing of the lot. In a batch operation, the physical location of the wafer in the batch processing equipment may influence uniformity of the processing effects across the lot. In an example where wafers are moving through a contaminated chamber, if the order in which each wafer is processed is known then the final wafer yield may be plotted against the processing order in this step. For each wafer in a lot a drop-off in yield versus processing order would be observed due to the contamination problem. This data is used to make adjustments to the line to improve yield; however, this wafer tracking method lacks the level of precision in the data collected required by chip plants today.
In tracking the wafer processing order, specialized equipment has been used to read scribed wafer identifiers, either immediately prior to or after critical processing steps, and to store this data for later correlation with device performance. Randomizing the order of the wafers prior to such steps is often done to ensure effects are not compounded. The wafer positional data is fed into a computer system, the device performance metrics for a wafer lot of interest are manually entered, and then all possible graphs of the device metrics for that lot versus wafer processing order at each step are generated. The data is then reviewed to determine those steps at which the processing order may affect performance. This type of approach to tracking wafers can be costly in its implementation due to the amount of hardware and software needed to randomize the wafer order and interface with the wafer processing system""s main computer database.
The present invention is directed to addressing the above and other needs in connection with improving wear rates of tooling and equipment in a semiconductor processing line and improving traceability of product as it moves through the manufacturing process.
According to one aspect of the invention, it has been discovered that by rotating a wafer to a defined angle of rotation and recording the rotation angle and the wafer""s corresponding location in the processing line improved accuracy in wafer tracking is achieved. It has also been discovered that tooling wear may be more controlled if the wafer being processed is moved axially prior to processing. Accordingly, a method of rotating a wafer in a multiple stage wafer processing system includes determining an incoming angle of rotation of the wafer as the wafer is presented to a first processing stage of wafer processing. As the wafer is exiting the first processing stage and moving into a second processing stage, the wafer is rotated to an outgoing angle of rotation. The angle of rotation and wafer location is recorded as a set of coordinates for each wafer as the wafer moves through each stage of wafer processing. In a related embodiment, a translation angle of rotation can also be imparted on the wafer while the wafer is within a stage of wafer processing.
According to another aspect of the invention, a system for rotating wafers in a multiple stage wafer processing system includes a scanning arrangement for determining an incoming rotation angle on a wafer as the wafer is presented to a first processing stage. A rotating apparatus rotates the wafer to an outgoing angle of rotation as the wafer is exiting the first processing stage and moving into a second processing stage. A computer arrangement records and tracks the angle of rotation and the corresponding wafer location as a set of coordinates for each wafer as the wafer moves through each stage of wafer processing.